From “The Tactics of Magic and Firearms” by Altos Wexan

The first firearms were dismissed as mere toys, especially after a single powerful warmage annihilated the entire Royal Rifle Corps in the Humbling on the Rhyn. But they had one important benefit: unlike the Art, which required years of study to master for all but the most talented, anyone could be trained in the use of firearms.

Most importantly, the advent of sealed brass cartridges and repeating firearms meant that guns were no longer limited to a single shot, and a full spread from a six-shooter could plow under an unprepared wizard in seconds. Considerable effort was devoted to bulletproof shield spells in retaliation, but these required concentration and considerable willpower to erect and maintain. It was also exceedingly difficult to fire spells from behind a shield.